Abstract

Very few studies exist on the processes and products of formerly ice-confined, multiple vent and multiple fissure basaltic ridge complexes, even though it is estimated that more than 1000 of these structures exist in Iceland. This is the first study that examines the processes, products and depositional environments of such complexes over space and time. The 21-km long, 2.5 km wide and 250 m thick Sveifluháls ridge in SW Iceland was used as a case study to better understand the nature of these complexes. Ridge growth began with eruptions beneath a 450-600 m thick ice sheet sometime between 43,000-12,400 years BP. The resulting melting ice and dropping overburden pressures facilitated the eruption of vitric phreatomagmatic tuff into one or more ice-confined meltwater lakes. These eruptions formed tuff cones, elongate tuff cones and narrow (0.5 km) tuff ridges, and collectively these formed a series of regularly-spaced (average spacing of 0.7 km) edifices along ~60 semi-parallel individual linear segments that vary in length from 0.25 km to 1.5 km. Tuff ridges represent edifices formed from Surtseyan fissure eruptions, and are described for the first time. All of the edifices grew by deposition from subaqueous density currents and/or by suspension and settling of phreatomagmatic tuff within the meltwater lakes. Tuff was commonly remobilized by slope failure due to melting and retreat of supporting ice walls, over steepening of the subaqueous tephra pile, and/or by disturbance from intrusions that propagated into wet tuff. The majority of tuff units at Sveifluháls are not in-situ, and about half of these contain chaotically-dipping beds that are suggestive of slumping and movement before final emplacement. Tuff cones and ridges grew to enclose inter-ridge catchments with volumes as large as 1.5x107 m3. These impoundments are interpreted as important sources for jökulhlaups. The total DRE volume of the ridge was ~2.0 km3 and an estimated total of 17.25 km3 of meltwater may have been generated during the eruptions of Sveifluháls. Prior to cessation of the eruptions, the ice-confined meltwater lake drained to 200 m below the original edifice height, and subaerial lava was erupted from at least one eruption center.